The present invention relates to a novel defoaming composition or, more particularly, to a silicone-based defoaming composition which is outstandingly stable even at a relatively high temperature and free from the troubles of flocculation in the foaming aqueous bath even by the addition of an inorganic salt in a high concentration to the foaming bath and unevenness in dyeing when the foaming liquid is a dyeing bath containing a dye.
As is well known, silicone-based defoaming agents in general, among various types of defoaming agents, have excellent characteristics that a remarkable defoaming effect can be obtained with a very small amount of addition of, e.g., 10 to 200 ppm, to the foaming liquid in addition to the advantages in the chemical inertness with excellent resistance against heat and chemicals not to react with the foaming substance under very adverse conditions. Moreover, silicone-based defoaming agents are free from the problem in the disposal of waste materials which sometimes cause serious environmental pollution. Therefore, silicone-based defoaming agents are widely employed in a variety of industrial processes by taking advantage of these features.
The above mentioned silicone-based defoaming agent is in the form of an oil compound prepared by compounding a silicone fluid such as dimethyl polysiloxane, methyl phenyl polysiloxane, methyl vinyl polysiloxane and the like with a finely divided silica filler or an aqueous emulsion prepared by dispersing the above mentioned oil compound in water by using a suitable surface active agent as an emulsifier.
Needless to say, defoaming of a dyeing bath is one of the most important applications of silicone-based defoaming agents. It is taught in Japanese Patent Kokai 49-25283 that a silicone-based defoaming agent particularly effective in defoaming of an aqueous dyeing bath can be obtained by compounding a finely divided silica filler, a first organopolysiloxane which is a dimethyl polysiloxane and a second organopolysiloxane which basically is a dimethyl polysiloxane but having at least one polyoxyalkylene group bonded to the silicon atom in the molecular chain terminal or at an intermediate position in the molecular chain. It is also known that an aqueous dyeing bath is sometimes admixed with an inorganic salt such as sodium sulfate and soda ash with an object to improve the efficiency of dyeing.
A problem in the above described dyeing process by the addition of a silicone-based defoaming agent to a dyeing bath containing an inorganic salt is that, when the concentration of the inorganic salt is increased or, for example, the concentration of sodium sulfate is in the range from 20 to 30 g/liter or higher, the constituents of the defoaming agent and the dye in the dyeing bath sometimes cause flocculation and separation from the aqueous medium resulting in unevenness in dyeing. This undesirable phenomenon is more remarkable at a higher temperature of the bath so that the dyeing process using a silicone-based defoaming agent is performed usually at 30.degree. to 45.degree. C. in order to avoid flocculation.
A proposal has been made in order to solve the above mentioned problem of flocculation in a dyeing bath at high temperatures in the presence of an inorganic salt by using copolymeric polyoxyethylene-polyoxypropylene groups as the polyoxyalkylene group in the above mentioned polyoxyalkylene group-substituted organopolysiloxane as the principal ingredient of the defoaming agent with a relatively high molar fraction of the oxyethylene units. A problem in this way of improvement is that the dispersibility of the finely divided silica filler in the silicone fluid is decreased by increasing the molar fraction of the oxyethylene units resulting in decreased stability of the emulsion by dilution or under a condition of mechanical working so that no practically useful defoaming agent can be obtained. Accordingly, it is eagerly desired to develop a novel and improved silicone-based defoaming agent having high stability and free from the problem of flocculation even by the addition of a large amount of an inorganic salt to the foaming liquid.